This SAOimage display shows the alignment of
HST Guide Stars with a test image with a world coordinate system.
immatch can be run to list matches
between HST Guide Stars and sources found in the image. The command used was

saogsc test.fts

where saogsc is

saoimage -reg `imgsc -q o -r 10 $1` -wcscom sgsc_-ha_%s $*

Clicking the w key on the star closest to the center returns the
following: